Method and system for determining the buffer action of a battery

ABSTRACT

A method for determining the buffering effect of the battery ( 2 ) for providing a voltage (U) for a power supply system ( 4 ), in particular for a vehicle, is provided in order to identify serviceability of a battery as easily and reliably as possible, in which method any voltage change and any current change are detected cyclically, the dynamic internal resistance of the battery is determined on the basis of the quotient of the voltage change and the current change, the specific dynamic internal resistance is monitored for exceeding an extreme value which can be predetermined, and a statement of the buffering effect of the battery is made and is output. In this case, the buffering effect of the battery is better, the smaller the quotient of the voltage change and current change.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of PCTApplication No. PCT/EP02/02032, filed on Feb. 26, 2002, which in turnclaims the benefit of the filing date of German Patent Application No.DE 10111408.7 filed on Mar. 8, 2001.

FIELD OF THE INVENTION

The invention relates to a method for determining the buffering effectof a battery for providing a voltage, in particular for a vehicle. Theinvention also relates to an arrangement for determining the bufferingaffect.

BACKGROUND OF THE INVENTION

Safety-relevant functions, such as electrical brakes, are beingincreasingly operated electrically in vehicles. In order to ensure thatthe serviceability of such safety-relevant components is guaranteed,they must be supplied with electrical power all the time. In order toachieve this, it is known for two energy sources to be provided in avehicle, namely a battery and a generator.

Since the life of a conventional lead-acid battery is generally shorterthan the life of a vehicle, it is possible for only one of the twoenergy sources to still be serviceable. This situation, in which abattery is discharged or defective and is thus unserviceable can resultin damage to the battery or overloading of the second energy source, thegenerator. This can lead to the supply to the loads no longer beingensured. Furthermore, the vehicle may change to a safety-critical state.For this reasons, measurements of the battery's buffering effect arerequired which on the one hand warn the driver and on the other handalso keep the vehicle power supply system, and hence the vehicle, in anoperationally safe state to ensure that loads which are relevant foroperation and in particular those which are relevant to safety, such asengine electronics, electrical brakes, are not switched off. In additionto ensuring that as little current as possible is drawn, a minimumvoltage level must be maintained in order to guarantee the operabilityof safety-relevant devices such as these. This can normally be ensuredby means of the generator. However, if the battery has beendeep-discharged or is defective in some other way, then it can no longerprovide a buffering effect. This may possibly lead to the generatorbeing de-energized by a load change during operation and the vehiclepower supply system voltage suddenly collapsing, so that it is no longerpossible to ensure that the safety relevant devices are operated.

In addition to ensuring that as little current as possible is drawn, aminimum voltage level must be maintained in order to guarantee theoperability of safety-relevant devices such as these. This can normallybe ensured by means of the generator. However, if the battery has beendeep-discharged or is defective in some other way, then it can no longerprovide a buffering effect. This may possibly lead to the generatorbeing de-energized by a load change during operation and the vehiclepower supply system voltage suddenly collapsing, so that it is no longerpossible to ensure that the safety relevant devices are operated.

A method for determining the buffering effect of a battery for providinga voltage for a power supply system for a vehicle is known from DE 19944 517 A1. In this case, the voltage ripple is detected and determined,and the maintenance of a limit value which can be predetermined ismonitored using a monitoring unit.

Furthermore, a method for testing the quality of a battery is known fromJapanese Laid-Open Specification JP 03-249 582 A. In this case, aconstant AC source is used as a load for the battery to be tested, andthe voltage level which then occurs is determined. The quality of thebattery is then assessed on the basis of the resultant voltage levels.However, owing to the use of the constant AC source, an arrangement suchas this cannot be used in a vehicle. Furthermore, the use of theconstant AC source represents an extra signal being fed in, and this iscomplex.

The invention is thus based on the object of specifying a method fordetermining the buffering effect of a battery, which allows theserviceability of a battery to be identified as easily and reliably aspossible. A further aim is to specify a particularly simple arrangementfor determining the buffering effect of the battery.

SUMMARY OF THE INVENTION

An aspect of the invention relates to a method to determine a bufferingeffect of a battery. The method comprises cyclically determining avoltage change and a current change generated by the battery;determining a dynamic internal resistance of the battery based on aquotient of the voltage change and the current change; determiningwhether the dynamic internal resistance exceeds a predetermined value;and performing a predetermined operation if the dynamic internalresistance exceeds the predetermined value.

Another aspect of the invention relates to an apparatus to determine abuffering effect of a battery. The apparatus comprises a first sensor togenerate a first signal related to a voltage generated by the battery; asecond sensor to generate a second signal related to a current generatedby the battery; a monitoring unit to generate a third signal related toa dynamic internal resistance of the battery using the first and secondsignals, and to cause a predetermined operation if the third signalexceeds a predetermined threshold.

The invention is in this case based on the idea that, when the batteryhas a negligible buffering effect, this leads to a change in the filtercharacteristics of the battery for the vehicle power supply system.According to the invention, the dynamic internal resistance of thebattery is determined for this purpose, on the basis of the quotient ofany voltage change and any current change. The dynamic internalresistance is preferably monitored for a minimum value which can bepredetermined (i.e., a first predetermined threshold). This makes itpossible to make a statement about the buffering effect of the batteryby monitoring the dynamic internal resistance for exceeding a minimumvalue. The smaller the quotient of the voltage change and the currentchange, the better is the buffering effect of the battery. The dynamicinternal resistance is in this case preferably determined when asignificant current or voltage change occurs, for example when a changeof more than 1 V or 5 A occurs.

In order to make a statement about the buffering effect of the batteryeven when small current and voltage changes occur, the internalresistance is preferably weighted by means of a weighting factor. Inparticular, this results in continuous monitoring of the dynamicinternal resistance, and hence the continuous identification of thebuffering effect. By way of example, the power is used as a weightingfactor, based on the following expression P=ΔU×ΔI. Depending on thenature and the embodiment, the weighting factor can be taken intoaccount as a linear function or square function when determining thedynamic internal resistance.

A warning message is advantageously output when the limit value, that isto say the minimum value, is exceeded. This ensures that a warningmessage is output to the driver, for example in the form of an indicatorlight on the dashboard, when the buffering effect of the battery is nolonger sufficient for emergency or safety operation or for short-termsevere overloading.

For this purpose, the apparatus according to the invention has, interalia, a monitoring unit for determining the dynamic internal resistanceof the battery on the basis of the quotient of any voltage change andany current change.

This and further objects, features and advantages of the presentinvention will become clear from the following detailed description ofpreferred exemplary embodiments of the invention in conjunction with thedrawing.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will be explained in more detailwith reference to a drawing, in which:

FIG. 1 shows, schematically, an arrangement for determining thebuffering effect of a battery, having a sensor unit and a monitoringunit,

FIG. 2 shows, schematically, the sensor unit,

FIGS. 3 to 4 show diagrams of a current/voltage characteristic for abattery, and

FIGS. 5A–5B to 8A–8B and 9A–9C to 12A–12C show diagrams with functionalprofiles of the voltage and current for various exemplary embodiments.

Parts which correspond to one another are provided with the samereference symbols in all the figures.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an arrangement 1 for determining the buffering effect of abattery 2 for providing a voltage U for a power supply system 4, inparticular for a vehicle which is not shown in any more detail. Thevehicle power supply system 4 is supplied with voltage U from thebattery 2. The vehicle power supply system 4 can be supplied withvoltage U from the vehicle generator 5 as a second energy source. Inaddition, a back-up battery bat2 (not shown) can be arranged in parallelwith the (vehicle power supply system) battery 2, and can be connectedin order to compensate for instabilities in the vehicle power supplysystem and for temporary charging when required.

The arrangement 1 has a means 6 for detecting and determining, as firstsignal, the voltage U and/or the voltage ripple UW (referred to forshort in the following text as a voltage sensor 6), and a monitoringunit 8 for monitoring the voltage U and/or the voltage ripple UW formaintenance of an extreme value G_(U), G_(UW) which can bepredetermined. In addition, the arrangement 1 has a means 10 fordetecting and determining, as a second signal, the current I and/or thecurrent ripple IW (referred to for short as a current sensor 10 in thefollowing text). The characteristic variables for the vehicle powersupply system 4, such as the voltage U, the voltage ripple UW, thecurrent I and/or the current ripple IW, are monitored by means of themonitoring unit 8 for compliance with an extreme value G_(U), G_(UW),G_(I) or G_(IW), which can be predetermined, in order to determine thebuffering effect of the battery 2. The monitoring is applied toovershooting and/or undershooting of the predetermined extreme valueG_(U), G_(UW), G_(I) or G_(IW), (that is, a second predeterminedthreshold). As the buffering effect of the battery 2 decreases, thevoltage ripple UW increases, and the current ripple IW decreases. Thevoltage ripple UW is caused in particular by switching on loads, forexample, the generator, the ignition system or a heated rear windshield11.

Thus, the voltage ripple UW is monitored in one exemplary embodiment.When monitoring the current ripple IW, the reciprocal behavior to thatof the voltage ripple is analyzed. If the respective predeterminedextreme value G_(UW) or G_(IW) is overshot or undershot during themonitoring of the voltage ripple UW and/or of the current ripple IW,then this is an indication that the buffering effect of the battery 2 isinadequate.

The monitoring unit 8 may in this case be in the form of hardwarecircuitry or software. By way of example, FIG. 2 shows a hardwarecircuitry version of the monitoring unit 8 for monitoring the voltageripple UW for compliance with the associated extreme value G_(UW). Inthis case, the monitoring unit 8 has a filter 12, in particular abandpass filter. The filter 12 filters low frequency components, inparticular the DC voltage, and high frequency components out of thevoltage signal U. Depending on the nature and embodiment of the filter12, this is a filter whose mid-frequency is derived from the rotationalspeed of the engine or of the generator 5. The filter 12 is followed bya rectifier 14 and a comparator 16. The oscillation which is filteredout of the voltage signal U by means of the filter 12 is converted bythe rectifier 14 to a DC voltage, and is compared by the comparator 16with the predetermined extreme value G_(UW). In other words: theamplitude of the voltage ripple UW is monitored by means of thepredetermined extreme value G_(UW) for a maximum value. If the DCvoltage downstream from the rectifier 14 is greater than the maximumvalue, then the buffering effect of the battery 2 is too low, and apredetermined operation may be performed (e.g., a warning message M isoutput, for example in visual form as “charge battery” or “replacebattery”, or audibly.

By analogy with the hardware circuitry monitoring unit 8 for the voltageripple UW, the current ripple IW is measured by tapping off the voltageU across a shunt resistance which is connected in series with thebattery 2, eg., a current-sensing voltage, which is filtered by a filter12. The filters used for filtering the voltage and current signals mayhave different time constants. In this case, the amplitude of thecurrent ripple IW is monitored for undershooting a predetermined minimumvalue. In addition to determining the ripples UW and/or IW, thebuffering effect can also be assessed by using the response of thevoltage U and current I, in particular when switching on a load, forcompliance with associated extreme values G_(U) and G_(I). A comparator16 based on hardware circuitry is likewise provided for this purpose.

In the exemplary embodiment according to the invention, the monitoringunit 8 may, for example, be in the form of a computer unit. Themonitoring unit 8 is in this case used for monitoring and assessing thevehicle power supply system variables, that is to say the voltage U, thevoltage ripple UW, the current I and/or the current ripple IW. By way ofexample, the computer unit is a microcontroller or some other dataprocessor unit. In order to take account of relationships between theripples, such as the current ripple IW and/or the voltage ripple UW, themonitoring unit 8 has families of characteristics, for example forrelationships between the voltage ripple UW and the temperature, thegenerator DC voltage, the generator current and the battery current I.These families of characteristics are stored, for example, in the formof curves or tables.

In the preferred exemplary embodiment of the invention, in addition tothe measurements of the voltage U and current I (e.g., the first andsecond signals) as described above, the dynamic internal resistance Ri(a third signal) of the battery 2 is determined using the monitoringunit 8, and is used to identify the buffering effect. Both the currentchange ΔI and the voltage change ΔU are determined in this case. Thequotient of the voltage change ΔU and of the current change ΔI is usedto determine the dynamic internal resistance Ri and to monitor forcompliance with a predetermined associated extreme value G_(R), inparticular for a minimum value being exceeded. If the minimum value isexceeded, then the buffering effect is no longer adequate. This meansthat, the smaller the quotient of the voltage change ΔU and the currentchange ΔI, the better is the buffering effect of the battery 2. Thedynamic internal resistance Ri is preferably determined when a minimumchange occurs in the current I and voltage U, for example when a currentchange ΔI of at least 5 A and a voltage change ΔU of at least 1 V occur.

The method according to the invention for determining the bufferingeffect of the battery 2 will now be described in more detail in thefollowing text.

In the method according to the invention, a new value pair for a voltageU and a current I from the battery 2 is determined cyclically, forexample every 4 ms. This results in the voltage change ΔU and thecurrent change ΔI. The monitoring device calculates filtered values fromthis value pair. The filter is, for example, a first-order low-passfilter. In this case, a high-speed filter is in each case preferablyformed, for example with T=8 ms for the voltage U and current I. If thevoltage is additionally filtered via a slow filter, for example in whichT=30 ms, the voltage and current profiles can be checked forplausibility, and voltage and current flanks can be selected in advance.

These filtered voltage and current values are then searched by themonitoring device for relative minimums and maximums. The dynamicinternal resistance Ri is determined after each identified extremevalue, that is to say relative minimum or maximum. However, this processof determining the dynamic internal resistance Ri is carried out onlysubject to the following boundary conditions:

-   -   Before the identified extreme value, start values resulting from        an opposite extreme value or the end of a maximum waiting time        for the next extreme value will have already been stored.    -   The sudden changes in the battery voltage from the high-speed        and slow filters have the same mathematical sign. Sudden changes        such as the falling flanks of a load dump or load shedding, when        the current decreases sharply, but the voltage does not change        to the same extent, are thus ignored.    -   The dynamic change to the battery voltage must have a specific        minimum magnitude. In the case of very large sudden voltage        changes and sudden changes which are completely in the discharge        area, it is sufficient to check that the slow voltage change is        not greater than the rapid change.    -   In the case of sudden changes in which the current starts or        ends in the discharge area, the high-speed sudden voltage change        must be 15–30% greater than the slow sudden voltage change.    -   Sudden changes which are completely in the charging area must        have the greatest dynamic response. The high-speed voltage        change must be 25–50% greater than the slow voltage change.    -   When the generator is running, this can be identified from the        fact that the sudden voltage change from the high-speed filter        must be greater by a specific factor than that of the slow        filter (for example 25%). For very large sudden voltage changes        (>2 V) or when the generator is inactive, it is sufficient to        check that the difference from the high-speed filter is at least        not less than that from the slow filter.    -   The mathematical sign of the sudden voltage change and sudden        current change must be the same (dU*dI>0), or the current must        be below the tolerance limit.

If these boundary conditions are satisfied, the dynamic internalresistance Ri is calculated.

Before calculating the dynamic internal resistance Ri, a check iscarried out to ensure that no division by zero is being carried out orthat a negative value has been determined for Ri. If the magnitude ofthe current is below the tolerance limit, the mathematical sign of thevoltage and a minimum value are transferred.

The significance of the identified flank is determined on the basis ofthe measurement error to be expected.

Based on the formula:

${Ri} = \frac{{dU} \pm {2 \times {fU}}}{{dI} \pm {2 \times {fI}}}$and taking account of the maximum resistance (measured voltage high,measured current small):

${significance} = {{abs}\left( \frac{1}{1 - \frac{{{dU} \times {dI}} + {2 \times {fU} \times {dI}}}{{{dU} \times {dI}} - {2 \times {fI} \times {dU}}}} \right)}$where fU and fI are the errors to be expected from the voltage andcurrent measurements respectively (for example noise, quantization errorfrom the A/D converter, . . . ).

The significance is thus derived from the relative error in theresistance determination (relative error=0 . . . 1).

The limit value analysis results for very large sudden voltage andcurrent changes with a significance which tends to infinity. For valuesfor which the sudden voltage change is large and sudden current changeis small, or for which the sudden voltage change is small and suddencurrent change is large, the resultant significance is small, and tendsto 0.

The significance, divided by 100 and limited to unity, results in theweighting. The dynamic internal resistance Ri, which is obtained fromthe quotient of the voltage change ΔU and current change ΔI, is weightedby means of this weighting factor.

A measured value with an expected error of less than 1% would thus betransferred immediately.

The rest of the calculation process takes account only of those valuesof the dynamic internal resistance Ri for which one of the followingconditions is satisfied:

-   -   The weighting indicates a sufficiently small measurement error.

-   The sudden voltage change is very large.

-   The sudden current change is very large.

-   The alternative calculation with a large sudden voltage change or    sudden current change means that it is also possible to calculate    very small or large resistance values (but with a high error    probability).

The new, filtered value of the dynamic internal resistance Ri isobtained from the old values and from the correspondingly weighted newvalue, using the formula:RiFilt=RiFilt×(1−weighting flank)+Riflank×weighting flank

In the initial phase, the sum of all the previous weighting processescarried out is less than unity. In this situation, the weighting for thecurrent flank is related to the sum of the existing weightings, and thisrelative value is used as the present flank weighting. The smaller thesum of the existing weightings and the higher the weighting for thepresent flank, the greater the extent to which this present flank isthus taken into account.

In addition to the normal calculation, a “high-speed dynamic internalresistance Ri-fast” is also calculated. This is composed of only thelast 20 calculated values and thus has a considerably faster dynamicresponse, and is less accurate, than the filtered dynamic internalresistance Ri. In the event of major discrepancies between the filtereddynamic internal resistance Ri and the high-speed dynamic internalresistance Ri-fast, the high-speed dynamic internal resistance Ri-fastis used, provided that it is sufficiently stable and that one of thevalues (filtered or high-speed) is above a specific minimum value. If novalues of the dynamic internal resistance Ri are determined for a verylong time, the high-speed dynamic internal resistance Ri-fast is reset,in order to produce values more quickly when stimulated once again.

The validity is assessed in measuring the time which is required tocalculate the high-speed dynamic internal resistance Ri-fast. If apredetermined time interval has passed here, the validity may be assumedto be no longer adequate.

A statement about the buffering effect of the battery 2 can be made fromthe validity and from the determined dynamic internal resistance Ri.

There are two output variables for the statement about the bufferingeffect of the battery 2:

-   1. A first output signal A1, which always produces a value, even    when the stimulus is no longer sufficient. This value is required in    order to ensure that a statement is always produced about the    buffering effect of the battery 2. If the stimulus is no longer    sufficient, this output signal remains at the last determined value.    New values are not determined again until the validity is    sufficient.-   2. A second output signal A2, which produces a value only when the    validity is sufficient. If this is no longer the case, the output    signal produces the value for “not available”.

The statement about the buffering effect of the battery is updatedwhenever a new dynamic internal resistance Ri has been determined andthe validity is sufficient.

The output variable is formed in two stages. Firstly, three areas areassessed independently of one another. The areas are then checked, andthe output variables are formed.

There are a number of critical areas in which the dynamic internalresistance Ri may move. These are assessed independently of one anotheron the same principle, with different parameters (thresholds and times).Each area has an associated upper and lower threshold as well as aminimum time for overshooting of the upper threshold and a minimum timefor undershooting of the lower threshold. In consequence, for very lowvalues (<20 mΩ), the dynamic internal resistance is calculated lessaccurately than, for example, at 80 mΩ. Furthermore, the criticalbattery state must be identified more quickly than, for example, duringexternal starting.

An upper threshold value Tho and a lower threshold value Thu are definedas thresholds (first predetermined threshold) for the dynamic internalresistance. In addition, two time intervals To and Tu, respectively aredefined, during which the thresholds must be exceeded by the dynamicinternal resistance Ri before the state for the corresponding areachanges. The time intervals may be set differently from each other,based on a state of a vehicle power supply system, an additionalmeasurement of the dynamic internal resistance, or an external start.

When a valid value for the dynamic internal resistance Ri is determinedfor the first time after resetting, a value is in each case assumed forthe buffering effect. This is Ri-lower if the value is below the upperthreshold value, otherwise it is Ri-higher. This procedure was chosensince, otherwise, no statement could be made for a relatively long timewhen the determined dynamic internal resistance Ri is between the twothreshold values, and the battery state is normally improving.Furthermore, no message is produced in the better case.

A change in the corresponding range variable then occurs only when:

-   The dynamic internal resistance Ri is below Thu for longer than Tu    when the state is poor; the buffering effect is then set to    Ri-lower.-   The dynamic internal resistance Ri is above Tho for longer than To    when the state is good; the buffering effect is then set to    Ri-higher.

Various thresholds and times are stated as the value range owing to thevarious battery sizes and technologies that are to be used, and theseare used to form individual range bits, as is shown in Table 1, so thatit is possible to distinguish between a threshold for a critical vehiclepower supply system state, a threshold for the initiation of additionalmeasures, for example connection of a second battery (back-up battery)bat 2, and a threshold for external starting fs.

TABLE 1 Tho [mΩ] Thu [mΩ] To [s] Tu [s] Comments Ri-crit 220–130 140–90 0.5–3   0.3–3   Threshold for critical vehicle power supply system stateRi-bat2 90–20 50–15  3–20  3–20 Threshold for initiating additionalmeasures, for example connection of a second battery Ri-fs 30–15 20–8 10–40 10–40 Threshold for external starting

Once the individual range bits have been formed, these are used togetherwith the information about the validity in order to form the outputvariables A1 and A2.

The logic linking of these range bits is shown in Table 2 below:

TABLE 2 Validity Ri-crit Ri-bat2 Ri-fs OK A1 A2 0 0 0 0 A1-io A2-nv 0 00 1 A1-io A2-io 0 0 1 0 A1-fs A2-nv 0 0 1 1 A1-fs A2-io 0 1 0 0 A1-bat2A2-nv 0 1 0 1 A1-bat2 A2-bat2 0 1 1 0 A1-bat2 A2-nv 0 1 1 1 A1-bat2A2-bat2 1 0 0 0 A1-crit A2-nv 1 0 0 1 A1-crit A2-crit 1 0 1 0 A1-critA2-nv 1 0 1 1 A1-crit A2-crit 1 1 0 0 A1-crit A2-nv 1 1 0 1 A1-critA2-crit 1 1 1 0 A1-crit A2-nv 1 1 1 1 A1-crit A2-critwhere A1-io is equivalent to the buffering effect being satisfactory,A1-fs is equivalent to the buffering effect in the external startingrange, A1-bat2 is equivalent to the buffering effect in the range inwhich the second battery should be connected, A1-crit is equivalent to acritical buffering effect, A2-io is equivalent to a satisfactorybuffering effect, A2-fs is equivalent to a buffering effect in theexternal starting range, A2-bat2 is equivalent to a buffering effect inthe range in which the second battery should be connected, A2-crit isequivalent to a critical buffering effect, and A2-nv is equivalent to avalue for the buffering effect not being available.

In order to provide sufficient stimulus for the process in phases inwhich a valid determination of the dynamic internal resistance Ri isrequired, one preferred development of the invention provides for theheated rear windshield to be used, if required, in order to produceadditional flanks. This is done by deliberately operating the heatedrear windshield, although the following boundary conditions must besatisfied in this case, to ensure that a stimulus is provided by theheated rear windshield:

-   The instantaneously determined value of the dynamic internal    resistance Ri must be greater than a specific value.-   The generator must be active.-   The instantaneous stimulus is not sufficient.-   A minimum time Tmin must have passed since the end of the last    stimulation pulse.

If these boundary conditions are satisfied, a drive device in themonitoring device first of all deactivates the heated rear windshield ifit is being operated. Once a delay time THSS off has elapsed, the heatedrear windshield is activated (again). A second delay time THSS on isthen allowed to pass, after which the heated rear windshield is returnedto normal control by the drive device.

This means that the dynamic internal resistance Ri can be determinedreliably and easily even in a situation in which the normal stimulus ismissing, so that a statement can be made without any problems about thebuffering effect of the battery 2.

The following text describes various measurements on the battery 2 withreference to diagrams, and explains their effects on the bufferingeffect of the battery 2 in more detail. By way of example, FIG. 3 showsa diagram of a current/voltage characteristic of a charged battery 2.The upper curve shows the voltage U, and the lower curve shows thecurrent I from the battery 2. The voltage ripple UW is very smalldespite the pulsed current change ΔI. The battery 2 thus has a largelygood buffering effect. In this case, as described above, the voltageripple UW is monitored by the arrangement 1 for compliance with thepredetermined limit value G_(UW).

By way of example, FIG. 4 shows a current/voltage characteristic for ameasurement with a battery 2 which has been discharged to a majorextent, and no longer has any buffering effect. The voltage ripple UW(upper curve) is considerably greater than that of the measurement inFIG. 3, and this is thus an indication that a buffering effect of thebattery 2 is insufficient.

In addition to determination and detection as well as monitoring of thevoltage ripple UW and/or of the current ripple IW, the behavior of thevehicle power supply system voltage U and of the vehicle power supplysystem current I, particularly when a load is connected, can also beused to assess the buffering effect of the battery 2, as alreadyexplained above.

FIGS. 5A and 5B show the voltage profile (top) and the current profile(bottom) while a load is switched on, for example the rear windshieldheating. The measurement was carried out without any battery 2, that isto say there was no buffering effect from the battery 2. Since there isno battery 2, no battery current I flows either, and all the current Imust be provided from a generator 5. The sudden rise in the current I inthe vehicle power supply system 4 first of all results in the voltage Ucollapsing, although a regulator within the generator regulates thevoltage U to its nominal value once again within about 600 ms.

The fact that the load has been switched on, in this case the rearwindshield heating, is in this case determined by means of the voltagesensor 6 and the current sensor 10 or, alternatively, by means of asignal via a databus. Once the load has been switched on, the voltagedip as determined by the voltage sensor 6 and the current sensor 10, andthe current profile are assessed by means of the monitoring unit 8,which is in the form of hardware circuitry and/or software. The currentprofile shows when the load is switched on, and the buffering effect isdeduced from the associated voltage profile. If the voltage U fallsbelow a predetermined limit value G_(U), for example of 12.5 V, thenthis indicates that the buffering effect of the battery 2 is no longersufficient.

The current change ΔI can also be used to estimate the current drawn bythe load. If the current change ΔI is considerably less than the ratedcurrent of the load, then the battery 2 can no longer supply sufficientcurrent I. In this case, the detection of the voltage U and of thecurrent I is carried out continuously, in particular immediately afterthe load is switched on, and within the regulation time of the generator5. For test purposes, the load can be switched on and off two or moretimes in order to ensure that the measurement does not have any otherload, switched on on a random basis, superimposed on it, with themeasurement providing incorrect statements.

As already explained in more detail above, the voltage change ΔU and thecurrent change ΔI can be used to determine the dynamic internalresistance. As already stated above, the quotient ΔU/ΔI is determinedfollowing a significant current or voltage change, for example of ΔU=1Vor of ΔI=5A, in the vehicle power supply system 4. In this case, themonitoring unit 8 monitors the quotient of the voltage change ΔU and thecurrent change ΔI for a predetermined limit value G_(R). The smaller thequotient of ΔU/ΔI, the better is the buffering effect of the battery 2.The monitoring therefore looks for the limit value G_(R), in particulara minimum value, being exceeded.

FIGS. 6A and 6B show the same measurement as that shown in FIGS. 5A and5B, but with a defective battery 2, that is to say a battery 2 with onlya limited buffering capability. The battery 2 is first of all chargeduntil the time t=10.25 s, after which the rear windshield heating isswitched on, and the current I then becomes negative. In this case, thebattery 2 does not supply all of the current I that is required tooperate the rear windshield heating. The sudden change in the batterycurrent I after the rear windshield heating or the load is switched onis less than the actual current (rated current) of the rear windshieldheating. In consequence, the voltage U from the generator 5 largelycollapses, that is to say down to U=12.5 volts. The generator 5 onceagain regulates the voltage U with an appropriate time constant. In thiscase, the monitoring unit 8 can use the high voltage ripple UW or thereduced sudden current change and the major dip in the voltage as acriterion that the battery 2 has a poor buffering effect.

FIGS. 7A and 7B show the current/voltage characteristic for the samemeasurement as that shown in FIGS. 6A and 6B, but with a battery 2having a better buffering capability than that battery. First of all,the battery 2 is charged until a time t=9.5 S, after which the rearwindshield heating is switched on and the current I becomes negative. Inthis case, more current I is drawn from the battery 2 for operation ofthe rear windshield heating than in the case shown in FIG. 6B. Thevoltage U from the generator 5 thus dips to a lesser extent, that is tosay down to U=13.25 volts. The generator 5 once again regulates thevoltage with the corresponding time constant. In this case as well, themonitoring unit 8 can use the high degree of voltage ripple UW or thereduced sudden current change and the major drop in voltage as acriterion for a poor buffering effect.

In contrast, FIGS. 8A and 8B show the current/voltage characteristic forthe same measurement with a battery 2 with a full buffering capability.This clearly shows that the voltage ripple UW is considerably less thanthat of FIGS. 5A, 5B to 7A, 7B. The battery 2 is first of all chargeduntil a time t=6.6 s, after which the rear windshield heating isswitched on. The current I becomes negative, and virtually all of thecurrent I for the rear windshield heating can be drawn from the battery2. In consequence, the voltage U from the generator dips by only about150–200 mV. The monitoring unit 8 uses the reduced voltage ripple UW todetermine that the battery 2 has a largely good buffering effect.

FIGS. 9A–9C to 11A–11C show the current/voltage characteristic as wellas the characteristic for the dynamic internal resistance Ri of abattery 2 which has been deep-discharged and which has been thawed out.In FIGS. 9A to 9C, the battery 2 is still severely frozen and is notabsorbing any charge, that is to say the current I=0. In this state, themonitoring unit 8 monitors in particular the dynamic internal resistanceRi, which is very high, with Ri being greater than 1 Ω. The battery 2therefore has no buffering effect. FIGS. 10A to 10C show thecharacteristics for the same battery 2 about 18.6 h after it has thawedout. In this case, the battery 2 is drawing current I. The dynamicinternal resistance Ri has fallen to a value of between 250 and 350 mΩ.By monitoring the dynamic internal resistance Ri, the monitoring unit 8thus identifies a buffering effect, since the dynamic internalresistance Ri has fallen below the associated limit value G_(R). Oncethe battery has been charged for a period of 7 minutes, the dynamicinternal resistance Ri falls to a value below 250 mΩ. The associatedcharacteristics are shown in FIGS. 11A to 11C.

FIGS. 11A to 11C show the current/voltage characteristic and thecharacteristic for the dynamic internal resistance Ri for a fullycharged battery 2 with a maximum buffering effect. The dynamic internalresistance Ri has fallen to a value of 10 to 20 mΩ.

Depending on the nature and the configuration of the arrangement 1, thevoltage ripple UW or the voltage drop can be monitored separately or ina combined manner in order to determine the buffering effect of thebattery 2. Furthermore, any other desired combinations of the monitoringof the current ripple IW, of the decreasing current I, of the voltagedrop U and/or of the dynamic internal resistance Ri can be monitored forcompliance with limit values G.

In summary, in order to identify the serviceability of a battery in assimple and reliable a manner as possible, the present inventiondiscloses a method for determining the buffering effect of the battery(2) for providing a voltage (U) for a power supply system (4), inparticular for a vehicle, in which a voltage change and a current changeare detected cyclically, the dynamic internal resistance of the batteryis determined on the basis of the quotient of the voltage change and ofthe current change, the determined dynamic internal resistance ismonitored for exceeding a limit value which can be predetermined, and astatement is made about the buffering effect of the battery, and isoutput. In this case, the buffering effect of the battery is better thesmaller the quotient of the voltage change and current change.

1. A method to determine a buffering effect of a battery, comprising:cyclically determining a voltage change and a current change generatedby said battery; determining a dynamic internal resistance of saidbattery based on a quotient of said voltage change and said currentchange; determining whether said dynamic internal resistance exceeds apredetermined value; performing a predetermined operation if saiddynamic internal resistance exceeds said predetermined value; filteringsaid voltage change and said current change; and determining saiddynamic internal resistance based on minimum or maximum values of saidfiltered voltage change and said filtered current change.
 2. The methodof claim 1, wherein filtering said voltage change and said currentchange comprises processing said current change and/or voltage change bytwo or more parallel filters having respectively different timeconstants, wherein respective outputs of said parallel filters are usedto enable said determination of said dynamic internal resistance.
 3. Themethod of claim 1, wherein enabling said determination of said dynamicinternal resistance using said respective outputs of said parallelfilters depends on a status of a vehicle power supply system.
 4. Themethod of claim 1, further comprising generating a weighting factor forsaid dynamic internal resistance based on a relative error ofdetermining said dynamic internal resistance.
 5. The method of claim 4,further comprising determining said dynamic internal resistance based onprevious measurements of said dynamic internal resistance modified bycorresponding weighting factors.
 6. The method of claim 5, wherein atmost the previous 20 measurements of said dynamic internal resistance isused to determine said dynamic internal resistance.
 7. The method ofclaim 5, wherein performing said predetermined operation is conductedwhenever a new dynamic internal resistance has been determined.
 8. Themethod of claim 1, further comprising: defining an upper threshold and alower threshold for said dynamic internal resistance; and defining afirst time interval and a second time interval during which said dynamicinternal resistance must respectively overshoot or undershoot to performsaid predetermined operation.
 9. The method of claim 8, furthercomprising setting said upper and lower threshold values and said timeintervals differently based on a state of a vehicle power supply system,an additional measurement of said dynamic internal resistance, or anexternal start.
 10. The method of claim 1, further comprising driving aload to said battery to make a valid determination of said dynamicinternal resistance.
 11. The method of claim 10, wherein said loadcomprises a heated rear windshield.